Gut microbiota and metabolite interface-mediated hepatic inflammation

Immunologic and metabolic signals regulated by gut microbiota and relevant metabolites mediate bidirectional interaction between the gut and liver. Gut microbiota dysbiosis, due to diet, lifestyle, bile acids, and genetic and environmental factors, can advance the progression of chronic liver disease. Commensal gut bacteria have both pro- and anti-inflammatory effects depending on their species and relative abundance in the intestine. Components and metabolites derived from gut microbiota–diet interaction can regulate hepatic innate and adaptive immune cells, as well as liver parenchymal cells, significantly impacting liver inflammation. In this mini review, recent findings of specific bacterial species and metabolites with functions in regulating liver inflammation are first reviewed. In addition, socioeconomic and environmental factors, hormones, and genetics that shape the profile of gut microbiota and microbial metabolites and components with the function of priming or dampening liver inflammation are discussed. Finally, current clinical trials evaluating the factors that manipulate gut microbiota to treat liver inflammation and chronic liver disease are reviewed. Overall, the discussion of microbial and metabolic mediators contributing to liver inflammation will help direct our future studies on liver disease.


Introduction
Liver inflammation accompanies most chronic liver diseases, such as alcoholic liver disease (ALD) and non-alcoholic fatty liver disease (NAFLD), accelerating their progression to liver fibrosis, cirrhosis, and primary liver cancers [1,2] .The term metabolic dysfunction-associated fatty liver disease (MAFLD) has been suggested to replace NAFLD recently, considering the criteria for disease diagnosis and heterogeneity of risk factors [3,4] .Both NAFLD and MAFLD in this review are used to keep consistent with the literature reports.Factors contributing to liver injury, such as alcohol consumption, pathogenic infection, and diet, can regulate liver inflammation.The gut-liver axis, a physiological crosstalk between the gut and liver via immunological and metabolic signals, can be regulated by dietary, genetic, epigenetic, and environmental factors [5,6] .Most metabolic diseases, such as obesity [7,8] , type 2 diabetes [9,10] , and cardiovascular diseases (CVDs) [11,12] , are associated with a disruption of the balance of gut microbiota (dysbiosis) leading to changes in intestinal permeability and systemic inflammation [13] .Accumulating evidence also shows that gut microbiota dysbiosis can specifically impact hepatic inflammation and metabolism in chronic liver diseases at different stages [14,15] .Hepatic parenchymal and non-parenchymal cells orchestrate liver inflammation, which, in turn, are also impacted by secreted inflammatory cytokines and chemokines from inflammatory immune cells.Hepatocytes account for 80% of liver parenchymal cells.Pattern recognition receptors (PRRs) expressed by hepatocytes can be activated by microbial-associated molecular patterns (MAMPs) and pathogen-associated molecular patterns (PAMPs) to produce inflammatory factors to activate liver resident cells [16,17] .The activation of liver resident immune cells (eg, liver resident macrophages or Kupffer cells) and infiltration of inflammatory immune cells (eg, monocytes and neutrophils) contribute to liver inflammation.Pro-inflammatory cytokines (eg, interleukin-1β or IL-1β and tumor necrosis factor-α or TNFα) secreted from activated inflammatory cells cause hepatocyte injury, while the secreted chemokines (eg, monocyte chemoattractant protein-1 or MCP-1/CCL2 and C-X-C motif chemokine ligand 1) chemoattract monocytes and neutrophils into the liver, accelerating liver inflammation [18,19] .
In this mini review, the roles of gut microbiota and gut microbiota-derived metabolites in the regulation of hepatic inflammation are summarized.The regulatory functions of different gut microbial genera or species in liver inflammation and the associated cellular mechanisms and molecular signaling pathways are reviewed.Finally, the potential strategies via modulating the homeostasis of gut microbiota to suppress liver inflammation are discussed, as well as the findings of current clinical trials.

The role of gut microbiota in liver inflammation
The dual roles of the gut microbiota, including both antiinflammatory and pro-inflammatory processes, are dependent on the relative abundances of gut microbiota and their species.Alterations of gut microbiota, such as an increase of pathogenic bacteria and a decrease of beneficial bacteria, are commonly associated with liver inflammation.

Pathogenic bacteria in liver inflammation
Intragastric administration of pathogenic bacteria Group A Streptococcus and Salmonella enteritidis advanced concanavalin A (ConA)-induced liver hepatitis in mice by activating liver immune cells, whereas depletion of intestinal Gram-negative bacteria mediated by oral treatment of gentamycin decreased ConA-induced liver injury by suppressing natural killer T (NKT) cell activation [20] .The relative abundance of Escherichia coli has been shown to be increased in patients with NAFLD compared with healthy controls, which can accelerate the development of hepatic steatosis, inflammation, and fibrosis.Mechanistic studies showed that flagella from E. coli can activate toll-like receptor 5 (TLR5) in liver sinusoidal endothelial cells (LSECs) to promote liver injury and epithelial to mesenchymal transition through the myeloid differentiation primary response gene 88 (MYD88)/ Twist1 signaling pathway [21] .In addition, high counts of E. coli were significantly and positively associated with hepatocellular carcinoma (HCC) in human patients [22] .Inhibition of bile acid (BA)-converting Clostridium species (eg, Clostridium scindens) that are responsible for converting primary BAs (PBAs) to secondary BAs (SBAs) can increase the expression of CXCL16 on LSECs to attract the infiltration of hepatic CXCR6 + NKT cells to inhibit liver cancer progression [23] .Another study showed that synergistic treatment of Bilophila wadsworthia promoted high-fat diet (HFD)-induced local and systemic inflammation (upregulation of interferon-gamma or IFN-γ and IL-6), intestinal barrier interruption, BA and glucose metabolism dysregulation, and hepatic steatosis.In contrast, the administration of a probiotic strain Lactobacillus rhamnosus CNCM I-3690 ameliorated B. wadsworthia-induced metabolic dysregulation and inflammation and strengthened intestinal barrier integrity [24] .Overall, a high abundance of pathogenic and inflammation-promoting bacteria can increase the circulating MAMPs and PAMPs to active PRRs in liver cells, which can induce and promote diet/ chemical/toxin-induced liver inflammation, injury, and fat accumulation.

Beneficial bacteria in liver inflammation
Patients with severe alcoholic hepatitis and prednisolone who received fecal microbial transplantation (FMT) from healthy donors through a naso-duodenal tube had a 90-day survival rate of 75% compared with 56% in the control group with only prednisolone [25] .FMT treatment reduced pathogenic bacteria such as Campylobacter (a microaerophilic bacterium) and anaerobic bacteria Parcubacteria, Weisella, and Leuconostocaceae, while increasing the abundance of Alphaproteobacteria and Thaumarchaeota [25] .Administration of Akkermansia muciniphila, a mucin-degrading bacterium, decreased HFD-induced body weight gain, hepatic steatosis, and liver injury in pathogen-free (SPF) male C57BL/6 mice.The hepatic protective effects of A. muciniphila were associated with the alteration of gut microbiota, including a decrease in the abundance of commensal bacteria Alistipes, Blautia, Butyricimonas, Lactobacilli, and Tyzzerella and an increase of potential beneficial bacteria Allobaculum, Anaeroplasma, Osclibacter, Ruminiclostridium, and Rikenella [26] .
Treatment of Lactobacillus gasseri strain CKCC1913 in diabetic mice reduced insulin resistance, fasting blood glucose levels, serum levels of pro-inflammatory cytokines (eg, TNF-α and IL-6), and hepatic oxidative stress-induced damage by increasing SOD activity and decreasing malondialdehyde levels.This therapy increased the abundance of beneficial bacteria such as Parabacteroides merdae and the production of short-chain fatty acids (SCFAs) to reduce liver oxidative stress and inflammation [28] .In vitro treatment of Lactobacillus sakei MJM60958 inhibited oleic acid and cholesterol-induced hepatocyte lipid accumulation.Additionally, in vivo administration of L. sakei MJM60958 suppressed HFD-induced NAFLD features in mice, including significant reduction of liver weight, body weight, and blood levels of ALT, AST, triglyceride (TG), urea nitrogen, and uric acid [29] .Treatment with human commensal L. rhamnosus GG5 protected against acetaminophen overdose and acute alcohol consumption-induced liver oxidative injury by producing 5-methoxyindoleacetic acid to activate nuclear factor erythroid 2-related factor 2 transcription factor [30] .
Ethanol feeding can induce gut dysbiosis by increasing the permeability of the gut barrier, relative abundance of pathogenic Escherichia and Staphylococcus, and liver inflammation, while depleting SCFA-producing bacteria, such as Prevotella, Faecalibacterium, and Clostridium [31] .In contrast, Pediococcus pentosaceus (strain CGMCC 7049) administration reduced ethanol-induced liver injury by decreasing serum ALT, AST, and TG levels, neutrophil infiltration, and the expression of inflammatory cytokines such as TNF-α and chemokines such as CCL2 and macrophage inflammatory protein-1α (MIP-1α/CCL3).In addition, P. pentosaceus treatment increased the expression of tight junction protein ZO-1, mucin proteins, and antimicrobial peptide Reg3β in the intestine.The relative abundance of SCFAproducing gut bacteria was also restored with P. pentosaceus administration, increasing propionic acid and butyric acid levels.
Furthermore, combined treatment with species such as Lactobacillus lactis and P. pentosaceus can reduce NAFLD activity score, improve gut-tight junction, alter gut microbial profiles (eg, Firmicutes/Bacteroidetes ratio), and restore the important metabolites such as SCFAs, BAs, and tryptophan metabolites to reduce systemic and locally liver and intestinal inflammation [32,33] .In summary, gut microbiota plays dual roles in liver inflammation, depending on bacterial species and their abundances (Table 1).

Gut microbiota-derived metabolites regulate liver inflammation at cellular and molecular levels
Gut microbiota-derived metabolites and components have broad effects on metabolic liver diseases [34,35] , accompanying liver inflammation.In this section, we review numerous gut microbial metabolites and their impact on liver inflammation at cellular and molecular levels.

Alcohol
Alcohol consumption is a driver for alcohol-related liver disease, which can result in the alteration of gut microbiota and increase intestinal barrier permeability [36,37] .Acute alcohol use in germ-free mice significantly increased hepatic inflammation and steatosis compared with conventional mice, including upregulation of alcohol-metabolizing enzymes in the liver [38] .Another murine study shows that alcohol is not metabolized by gut microbiota directly, but alcohol consumption increases Chen et al [20]   Non-alcoholic fatty liver disease (NAFLD) Escherichia coli E. coli has been shown to accelerate the development of hepatic steatosis, inflammation, fibrosis, and primary liver cancer.

Clostridium species such as Clostridium scindens
Inhibition of bile acid (BA)-converting Clostridium species that are responsible for converting primary BAs to secondary BAs can increase the expression of CXCL16 on LSECs to attract the infiltration of hepatic CXCR6 + NKT cells to inhibit liver cancer progression.
Ma et al [23]   Hepatic steatosis and systemic inflammation

Bilophila wadsworthia
Promoting high-fat diet (HFD)-induced local and systemic inflammation, intestinal barrier interruption, BA, and dysregulation of glucose metabolism, and hepatic steatosis, which can be inhibited by treatment of a probiotic strain Lactobacillus rhamnosus.
Jiang et al [28]   NAFLD and hepatocyte steatosis In vitro and in vivo treatment of Lactobacillus sakei MJM60958 Inhibition of oleic acid and cholesterol-induced hepatocyte lipid accumulation in vitro and suppression of HFDinduced NAFLD features in vivo.
Nguyen et al [29]   Acetaminophen overdose and acute alcohol consumption-induced liver oxidative injury
Jiang et al [31]   NAFLD Lactobacillus lactis and P. pentosaceus A combined treatment can reduce NAFLD activity score, improve gut-tight junction, alter gut microbial profiles (eg, Firmicutes/Bacteroidetes ratio), and restore the important metabolites such as short-chain fatty acids (SCFAs), BAs, and tryptophan metabolites to reduce systemic and locally liver and intestinal inflammation.
the circulating acetate production to result in gut microbiota alteration in mice [39] .
Endogenous ethanol is positively associated with the development of non-alcoholic steatohepatitis (NASH) in human patients with an increased abundance of alcoholproducing bacteria, such as phylum Proteobacteria, family Enterobacteriaceae, and genus Escherichia [40] .Both drinking and endogenous ethanol can promote liver inflammation by regulating gut microbiota.
Both innate and adaptive immune responses play important roles in the pathogenesis of ALD.Alcohol-induced damage of the gut barrier can lead to the increase of gut microbial components such as LPS in the portal vein circulation to activate intrahepatic innate immune cells (eg, macrophages) via TLR4, which induces liver inflammation and expression of many pro-inflammatory cytokines (eg, CCL2) [41] .Meanwhile, impaired cytotoxic T cells, a decrease of regulatory T cells (Tregs) and B cells, and an increase of T H 17 cells and CD57 + T cells are shown in patients with alcoholic steatohepatitis [41][42][43] .

Indole and tryptophan derivatives from gut microbiota
Indole is a microbial metabolite produced from the aromatic amino acid tryptophan [44] .Gut-derived indole and its derivatives can regulate intestinal barrier integrity, immunity, and gut hormones to impact hepatic inflammation and energy metabolism [45]   .Oral administration of indole in mice decreased LPS-induced liver inflammation by reducing hepatic pro-inflammatory gene expression (IL-1b, IL-6, and IL-15) and ameliorated oxidative stress by downregulating the expression genes Nos2 (nitric oxide synthase 2) and Nox2 (NADPH oxidase 2) [46] .Ex vivo experiments using the model of precision-cut liver slices demonstrated that the anti-inflammatory effect of indole is mediated by the suppression of nucleotide-binding domain and leucine-rich repeat-containing family pyrin domain-containing 3 (NLRP3) signaling pathway [46] .Indole-3-acetic acid (IAA) is another gut microbiota-derived metabolite from dietary tryptophan.Intraperitoneal injection of IAA at a dose of 50 mg/kg bodyweight in mice alleviated HFDinduced insulin resistance, high fasting blood glucose levels, and liver total TGs and cholesterol [47] .In addition, IAA treatment decreased hepatic inflammation and oxidative stress by reducing F4/80 + macrophage infiltration, the expression of TNF-α and MCP-1/CCL2, and the production of reactive oxygen species, malonaldehyde, and glutathione, as well as the SOD activity in liver tissues [47] .
Metabolites tryptamine and indole-3-acetate (I3A) are produced by gut microbiota, which are absent from germ-free mice and can be deprived by HFD treatment.Treatment with both metabolites can decrease fatty acid (palmitic and oleic acids)and LPS-stimulated expressions of pro-inflammatory cytokines in macrophages and inhibit chemokine (CCL2)-induced macrophage migration.In addition, I3A treatment can reduce fatty acid and/or TNF-α-induced hepatocyte inflammation by decreasing the expression of fatty acid synthase and sterol regulatory element-binding protein-1c [48] .In summary, indole and tryptophan-derived metabolites can suppress liver inflammation.

Linoleic acid (LA)
Oral treatment of Lactobacillus reuteri reversed ethanol-induced hepatitis, inflammatory cell infiltration, and lipid accumulation through the regulation of fatty acid metabolic pathways, such as alpha-linolenic acid and linoleic acid (LA) metabolism pathways [49]   .Bacteria such as Lactobacillus and Bifidobacterium can convert LA to conjugated linoleic acid (CLA) [50,51] .Treatment of CLA significantly decreased the mRNA expression of liver TNF-α, IFN-γ, and IL-1β and increased the mRNA and protein expression of intestinal tight junction proteins (occludin and ZO-1) in ob/ob (obese) mice [52] .In addition, CLA treatment in these mice increased the abundance of beneficial bacteria, such as Lachnoclostridium, Roseburia, Dubosiella, Oscillibacter, and Anaerostipes, and decreased the abundance of pro-inflammatory bacteria, such as Tyzzerella and Alistipes.However, CLA treatment displayed different functions in wild-type mice, which can induce hepatic inflammation and increase intestinal permeability [52] .Another study also showed that pregnant rats with an LA-rich diet predispose offspring to develop hepatic steatosis and subsequent metabolic disorders [53] .The underlying cellular and molecular mechanisms of LA-and CLA-mediated effects in liver inflammation need to be further studied.

Phenylacetic acid (PAA)
Phenylacetic acid (PAA) is a gut microbiota-dependent metabolic derivative of dietary phenylalanine, produced mainly by three gut microbiota phyla, including Bacteroidetes, Firmicutes, and Proteobacteria [54] .PAA can be further conjugated to glutamine to form phenylacetylglutamine (PAGln) in primates or to glycine to form phenylacetylglycine in rodents.PAGln has been shown to be positively associated with CVD and its associated morbidity and mortality by interacting with G-protein coupled receptors, including α2A, α2B, and β2-adrenergic receptors [55] .

Pentadecanoic acid
The increase of inulin-induced commensal Parabacteroides distasonis can increase the production of pentadecanoic acid, restore gut barrier integrity, and suppress hepatic steatosis and inflammation by decreasing serum levels of LPS and hepatic proinflammatory cytokines (eg, TNF-α and IL-6) in mouse NASH models [56] .Another study revealed that treatment with a combination of live Bifidobacterium, Lactobacillus, Enterococcus, and Bacillus can ameliorate cyclophosphamide-induced rat death, weight loss, and gut, liver, spleen, and lungs damage by regulating gut microbiota profiles, including an increase in phylum levels of Proteobacteria, Fusobacteriota, and Actinobacteriota and a decrease of Spirochaetota and Cyanobacteria [57] .

Short-chain fatty acids (SCFAs)
C57BL/6 mice fed with a diet containing 5% ethanol had increased hepatic fat accumulation and serum markers of liver injury, such as ALT and AST.This feeding alcohol diet caused intestinal barrier disruption and gut dysbiosis with an increase in the relative abundance of pathogenic bacteria Escherichia and Staphylococcus and depletion of SCFA-producing bacteria, including Prevotella, Faecalibacterium, and Clostridium [31] .In contrast, administration of P. pentosaceus, an ethanol-resistant probiotic strain, increased the production of SCFAs (propionic acid and butyric acid) by inhibiting an alcohol-induced decrease in the relative abundance of gut microbiota Lactobacillus, Pediococcus, Prevotella, Clostridium, and Akkermansia [31] .This protective effect also promoted the expression of intestinal tight junction proteins, mucins, and antimicrobial peptides [31] .Supplementation of Bifidobacterium breve and Bifidobacterium longum ameliorated the progression of NAFLD in mice by reducing the Firmicutes/Bacteroidetes ratio in the gut microbial profiles and increasing the production of SCFAs (acetic acid, butyric acid, and propionic acid) and tryptophan metabolites (eg, indole-3-propionic acid and -acrylic acid) [58] .
However, SCFAs may regulate immune responses to increase the development of HCC.Feeding a fermentable fiber diet can cause silent portosystemic shunt and cholemia in mice by increasing the production of SCFAs and BAs, predisposing liver injury and progression of cholestatic HCC [59] .Studies also found that the levels of SCFAs (eg, butyrate) and their intermediates were increased in the feces and sera of patients with NAFLD-HCC compared with patients with NAFLD-cirrhosis and non-NAFLD controls [60] .Mechanistically, the bacterial extract from patients with NAFLD-HCC compared with that from patients with NAFLD-cirrhosis and non-NAFLD controls significantly increased the frequency of Tregs but decreased the frequencies of CD8 + T cells, CD14 + monocytes, and CD19 + CD20 + B cells in peripheral blood mononuclear cells from non-NAFLD controls [60] .

Secondary bile acids (SBAs)
SBAs are produced by gut microbiota from PBAs that are synthesized in the liver [61] .Oral gavage of live A. muciniphila can decrease the production of SBAs in the cecum, such as deoxycholic acid (DCA) and lithocholic acid (LCA), to ameliorate hepatic steatosis and inflammation in mice with HFD-induced MAFLD [26] .The serum levels of SBAs were positively and significantly associated with liver fibrosis in patients with histologically approved NAFLD, especially for patients with mild fibrosis [62]   .Blocking DCA production or decreasing gut microbiota that produces DCA can prevent HCC development in obese mice by reducing senescence-associated secretory phenotype in hepatic stellate cells [63] .BAs such as chenodeoxycholic acid (a PBA) and DCA (a SBA) can induce IL-1α and IL-1β secretion in LPSprimed one marrow-derived dendritic cells in vitro and regulate the migration of neutrophils and monocytes in vivo [64] .

Taurocholic acid
The gut microbiota composition in mice with a high-cholesterol diet changed during the development of hepatic steatosis, steatohepatitis, and HCC, including a sequential increase of Mucispirillum, Desulfovibrio, Anaerotruncus, and Desulfovibrionaceae and a depletion of Bifidobacterium and Bacteroides.This phenomenon was also shown in human patients with hypercholesterolemia [65] .Gut bacterial metabolites taurocholic acid and 3-indolepropionic acid were increased and decreased in these mice, respectively.

Trimethylamine N-oxide (TMAO)
Trimethylamine N-oxide (TMAO) is a metabolite of dietary choline, betaine, or carnitine generated by gut microbiota, which can aggravate hepatic TG accumulation in mice and increase lipogenesis in palmitic acid-treated HepG2 cells.TMAO-induced lipid production is mediated by upregulation of the synthesis of BAs that can activate the Farnesoid X receptor signaling pathway [66] .
Overall, gut microbiota-derived metabolites can impact liver inflammation and disease progression (Table 2).Treatments that increase the favorable gut microbiota or suppress the unfavorable bacteria can inhibit liver inflammation, and vice versa.

Factors regulate the change of gut microbiota and their metabolites to impact liver inflammation
Factors including lifestyle, environment, medicines, hormones, and genetics impact gut microbiota profiles and their metabolites to aggregate or dampen hepatic inflammation and injury.In this section, we discuss the latest updates on how these factors impact gut microbial profiles and liver inflammation.

Diet and drink
Energy (eg, lipid and sugar) metabolism can significantly impact the profiles of gut microbiota and their metabolites.For example, alterations in glucose, fatty acid, and lipoprotein metabolism are commonly associated with NAFLD and obesity in mice and humans [67] , resulting in systemic and local liver inflammation.Insulin plays an important role in the regulation of energy metabolism in tissues such as adipose tissue, liver, and intestine, which can be impacted by gut microbiota and their metabolites [68,69]   .Insulin resistance increases the circulating levels of glucose and drives hepatic de novo lipogenesis to aggregate NAFLD progression [70] .

Exercise
Exercise can reduce obesity, metabolic syndrome development, and hepatic steatosis in HFD-fed rats.Exercise further improves the homeostasis of BA synthesis and reverses diet-induced imbalance of gut microbiota by increasing gut microbial genera Parabacteroides, Bacteroides, and Flavobacterium and decreasing genera Blautia, Dysgonomonas, and Porphyromonas [79]   .Aerobic exercise can also restore intestinal tight junctions, suppress LPS production and LPS-binding protein expression, and reduce liver inflammation by inhibiting LPS/TLR4/nuclear factor (NF)-κB signaling pathway [80] .In patients with NAFLD, exercise can significantly reduce body mass index (BMI), plasma TGs and apolipoprotein B, visceral fat area, body fat mass, and intrahepatic lipid content, when comparing responders with non-responders post 12-week intervention.One of the underlying mechanisms is that exercise significantly restructures the gut bacteria interactome, which benefits metabolic balance [81] .In addition to the improvement of hepatic fat accumulation and metabolism, exercise can also regulate the function of liver resident macrophages to modulate liver inflammation and fibrosis [82]   .In addition, exercise can improve mitochondrial function to enhance aerobic metabolism [83] .However, exercise can also directly cause abnormal liver function tests in healthy young adults [84] .For example, strenuous exercise (vigorous exercise) increases hepatocyte permeability inducing an upregulation of liver enzymes [84] .

Socioeconomic and environmental factors
Socioeconomic factors such as income, education, employment, and social communication can impact the profiles of gut microbiota [85,86] .One study showed that there was a significant association between the number of socioeconomic parameters and prediction of NASH (four or more) or severe steatosis (six or more), but not advanced fibrosis [87] .These socioeconomic factors include employment status, education degree, percentage of foreign born, private vs public health care, percentage without a car, liver environment, and so on.Another study also showed that the incidence rate of ALD was negatively correlated with the education level [88] .In relation, air pollution has been shown Zhu et al [40]   Liver inflammation Indole Suppress LPS-induced liver inflammation by inhibiting nucleotide-binding domain and leucine-rich repeat-containing family pyrin domain-containing 3 (NLRP3) pathway.
Krishnan et al [48]   Alcoholic liver disease (ALD) Alpha-linolenic acid and linoleic acid Oral treatment of Lactobacillus reuteri reversed ethanol-induced hepatitis, inflammatory cell infiltration, and lipid accumulation through regulation of fatty acid metabolic pathways, such as alpha-linolenic acid and linoleic acid metabolism pathways.

Pentadecanoic acid
The increase of commensal Parabacteroides distasonis induced by inulin can increase the production of pentadecanoic acid and restore gut barrier integrity and hepatic steatosis and inflammation in mouse NASH models.

Oral gavage of live
A. muciniphila can decrease the production of SBA in the cecum, such as DCA and LCA to ameliorate hepatic steatosis and inflammation in mice with HFD-induced MAFLD.
Wu et al [26]   NAFLD and mild liver fibrosis

SBAs
The serum levels of SBAs were positively and significantly associated with liver fibrosis in patients with NAFLD, especially for NAFLD patients with mild liver fibrosis.
Liu et al [62]   Hepatic steatosis or lipid accumulation Trimethylamine N-oxide (TMAO) TMAO can aggravate hepatic triglyceride accumulation in mice and increase lipogenesis in palmitic acid (PA)-treated HepG2 cells, which can upregulate the synthesis of bile acids.
Yang et al • 2024 (6):1 Immunometabolism to impact the composition of gut microbiota in rodent models and metabolic signaling pathways [89][90][91] , which may impact liver disease.For example, a study showed that long-term exposure to NO 2 impacts liver function in patients with schizophrenia by significantly increasing the levels of gamma-glutamyl transpeptidase and glutamic pyruvic transaminase [92] .Gut microbial phyla including Firmicutes, Actinobacteria, and Proteobacteria played an intermediary role in this process [92] .

Drugs
Antibiotics are the most common treatments for bacterial infections.The "double-edged sword" effect of antibiotic treatment with resultant gut microbiota changes has been illustrated in the last century [93] .For example, treatment with an antibiotic cocktail consisting of vancomycin, neomycin, and primaxin alters gut commensal bacteria, which can suppress HCC development via regulation of hepatic NKT cell function and IFN-γ production upon antigen stimulation [23] .In contrast, gut microbiota plays an important role in drug-induced hepatotoxicity.Treatment with the anti-cancer drug cisplatin can induce liver inflammation and cell apoptosis while also increasing the relative abundance of gut microbiota, such as Escherichia, Parabacteroides, and Ruminococcus.In addition, ablation of gut microbiota by antibiotics can protect against anti-cancer drug cisplatin-induced liver cytotoxicity [94] .Meanwhile, studies also show that antibiotic treatment can compromise immune checkpoint inhibitor efficacy, correlating with lower objective response rates, overall survival, and progression-free survival [95] .

Hormones
Melatonin, a hormone produced in the brain in response to darkness, can reduce aflatoxin B1-induced liver injury via the suppression of TLR4, MyD88, phospho-NF-κB (p-p56), and p-IκBα (nuclear factor of kappa light polypeptide gene enhancer in B cells inhibitor-alpha) [76] .However, melatonin may lose its function in mice treated with antibiotics [76] .Another study showed that melatonin treatment significantly reduced the relative abundance of gut bacterial genera Lactobacillus and Desulfovibrio and increased the abundance of genera Bacteroides, ameliorating ochratoxin A-induced liver inflammation and restoring gut barrier function [96] .

Genetic factors
Sirtuin 2 (SIRT2) deficiency promoted lipid deposition and inflammation in cells accompanied with HFCS (high-fat/ high-cholesterol/high-sucrose)-induced obesity, hepatic steatosis, and an aggravated metabolic profile, indicating SIRT2 deficiency advances NAFLD-NASH progression.Further study indicated that SIRT2 deficiency induced gut microbiota dysbiosis, inducing a decrease of bacterial genera Bacteroides and Eubacterium and an increase of genus Acetatifactor compared with the gut microbial profiles in wild-type mice [97] .In addition, the profiles of serum metabolites changed in mice with SIRT2 knockout, with an upregulation of l-proline and downregulation of phosphatidylcholines, lysophosphatidylcholine, and epinephrine.The expression of SIRT2 was downregulated in human patients with NALFD compared with healthy controls, accompanying the progression of NAFLD to NASH [97] .Another study showed that SIRT1 deficiency in the intestine in mice with bile duct ligation can cause intestinal inflammation by increasing macrophage infiltration and cytokine expression (eg, TNF-α and IL-6) while limiting the production of SCFAs [98] .
Overall, given the important roles of gut microbiota-derived metabolites in systemic and liver inflammation, manipulations of gut microbiota with different strategies are potential options for the suppression of inflammation, both locally and systematically.
In addition, there are many recruiting clinical trials that aim to evaluate the effects of gut microbiota regulation on liver diseases (Clinicaltrials.gov,such as NCT05006430 and NCT04932577) (Table 3).However, some treatments do not show promising effects.For example, the use of probiotic supplements (Lactobacillus acidophilus ATCC SD5221 and Bifidobacterium lactis HN019) alone for 6 months only improved the AST to platelet ratio index score in patients with NASH but did not change liver enzymatic markers, inflammatory parameters, hepatic steatosis and fibrosis, and gut microbiota significantly [105] .Another clinical trial (NCT03127696) showed that repeated FMTs can increase the level of microbiota engraftment and its duration in patients with obesity and type 2 diabetes.In addition, FMT plus lifestyle modification can increase beneficial microbiota in recipients to improve lipid metabolism and liver stiffness [112] .Furthermore, a mixed probiotic treatment (NCT04074889) with multiple strains containing six different Lactobacillus and Bifidobacterium species can improve mucosal immune function to reduce intestinal permeability in patients with NAFLD [103] .Overall, these results suggest that synergistic treatments may improve the efficacy of therapeutics.

Conclusions
Liver inflammation is commonly associated with metabolic liver diseases, such as ALD, NAFLD or MAFLD, liver fibrosis, cirrhosis, and cancers.Accumulating evidence shows that gut microbiota can impact liver inflammation in various liver diseases through the gut-liver axis, the physiological crosstalk between the gut and liver via immunological and metabolic signals.This axis can be regulated by dietary, genetic, and environmental factors.Factors including lifestyle, environment, medicines, hormones, and genetics can impact the Rodriguez et al [110]   NCT02637115 N/A Akkermansia muciniphila Reducing insulinemia and plasma total cholesterol and improving insulin sensitivity in patients with obesity-related metabolic disorders.
Depommier et al [111]   NCT03127696 N/A FMT Repeated FMTs can increase the level of microbiota engraftment and its duration in patients with obesity and type 2 diabetes.In addition, FMT plus lifestyle modification can increase beneficial microbiota in recipients to improve lipid metabolism and liver stiffness.

Lactobacillus and
Bifidobacterium species This mixed probiotic can improve mucosal immune function to reduce intestinal permeability in patients with NAFLD.
gut microbial profiles and their metabolites to aggregate or dampen liver inflammation and injury (Figure 1).Clinical trials have been performed or are recruiting to test the application of FMT, drugs (eg, anti-alcoholism drug disulfiram), diets, probiotics, synbiotics, antibiotics, fibroblast growth factor 19 analog (aldafermin or NGM282), and physical activity for the treatment of liver inflammation and gut microbiota dysbiosis.Current clinical trials show that a single treatment alone (eg, probiotics) cannot significantly reduce liver inflammation by regulating the gut-liver axis.Synergistic treatment or a combined treatment of microbial cocktail may be an option to improve the treatment efficacy via regulating gut microbial profile to suppress liver disease progression.Meanwhile, the underlying mechanisms causing the failure of the treatments in patients remain to be clarified.

Figure 1 .
Figure 1.The roles of gut microbiota and metabolites in liver inflammation and potential regulatory factor.Gut microbiota dysbiosis impacts liver inflammation, accompanying the infiltration of pro-inflammatory cells (eg, macrophages and neutrophils), secretion of inflammatory cytokines (eg, TNF-α, IL-1β, and IFN-γ), hepatic cell apoptosis, lipid accumulation, and oxidative stress.Alteration of gut microbiota including the increase of pathogenic bacteria and decrease of beneficial bacteria can change the products of secondary bile acids and metabolites such as alcohol and indole metabolism to promote liver inflammation.Factors including diet, drink, exercise, stress, genetic factors, drugs, and hormones can regulate the change of gut microbiota to treat liver inflammation.IFN-γ, interferon-γ; IL-1β, interleukin-1β; TNF-α, tumor necrosis factor-α.The figure was prepared using Biorender (https://biorender.com).

Table 1
Functions of bacterial supplementation in liver disease.Aggregation of ConA-induced liver hepatitis via activating liver natural killer T (NKT) cell activation in mice.

Table 2
Gut microbiota-derived metabolites in liver disease and inflammation.

Table 3
Clinical trials of modulating gut microbiota to treat liver inflammation and chronic disease.Leading to hepatic encephalopathy in patients with cirrhosis, systemic inflammation, and repaired intestinal barrier via reducing levels of mucin-degrading sialidase-rich species, such as Veillonella in a dose-dependent manner, which might reduce the toxic bile acids in patients with NASH.-month physical activity intervention plus an inulin-enriched diet decreased BMI, liver enzymes, and plasma cholesterol, and improved glucose tolerance in subjects with obesity, and more physical exercise can significantly increase inulin-induced regulation of microbial genera